Known apparatuses for checking dimensions and/or shape errors of workpieces comprise one or more feelers that touch the surface of the piece to be checked and are coupled to a housing by means of armsets allowing them to perform limited displacements. The displacements of the feelers, detected by transducer and/or display devices, for instance inductive transducers, or mechanical gauges, typically indicate variations of the checked dimension with respect to a nominal value. Mechanical transmission assemblies may comprise an axially movable shaft that has one end in contact with the feeler/s and the other one with a movable element of the transducer device.
Such an apparatus is shown, for instance, in U.S. Pat. No. 4,170,831 which makes specific reference to a so-called “plug” comparator for checking hole diameters, with a housing, two transversally movable feelers and a mechanical transmission assembly with a shaft having at an end thereof proper sloping surfaces in contact with spherical surfaces solid with the feelers. The shaft is axially movable within the housing and features, at the opposite end, a transversal surface that is kept in contact with a movable element of a dial indicator that is also connected to the housing. The axial displacements of the shaft are guided by a mechanism featuring, for instance, mutually coupled cylindrical surfaces defined by the shaft and the housing, and can include proper sliding bushings, ball bearings or similar known devices.
It is generally difficult to correctly dimension the mechanism guiding the shaft while assuring the right coupling between the feelers and the end of the shaft. In fact, in case that a guide is chosen constraining the axial displacement substantially free of play, the coupling with the feelers may be improper, and, besides, the unavoidable transversal stresses due to the radial displacement of the feelers may give rise to jamming and seizures. On the other hand, if a not negligible backlash is present, so that a better coupling with the feelers is guaranteed, it is possible that the shaft lean with respect to the axial displacement direction: in the latter case, even minor angular displacements—improper and unnoticed—of the transversal surface that is in contact with the dial indicator, cause the latter to provide wrong indications.
The need to have a mechanical transmission assembly properly sized is ever increasing when the required precision increases, for instance in case that the dial indicator is replaced by a more sensitive inductive or optical transducer, or a transducer of a different kind. Additionally, it is economically disadvantageous to employ high accuracy items in apparatuses having a simple construction and low cost components.
The problem is most acute in comparators having three feelers arranged, for instance, at 120° one from the other, where it is very difficult to reach an acceptable compromise solution by means of the known systems. In fact, it is particularly complex to guarantee the proper coupling between the end of the shaft and all three feelers, that can move each independently from the others, and in the meantime to have the shaft guided with an acceptable backlash.
There are also known comparators, for instance plug comparators like the ones shown in U.S. Pat. No. 5,746,003, where the feelers directly act on an axially movable part of a transducer with neither separate transmission devices therebetween, nor guiding mechanisms distinct from and additional to the guiding parts that are inside the transducer as a part of the latter. Even though some resilient portions or elements are connected to the transducer to facilitate the transmission of the displacements of the feelers, the assembling procedure of the comparator, involving such resilient portions/elements, needs particularly high precision and care and, as a consequence, is lengthy and burdensome. Moreover, the axial guide of the movement is entirely left to the guiding parts of the transducer, the latter—that is a delicate device per se—being so susceptible to unusual transversal stresses that may cause jamming and/or breakages.